/*
 * Copyright (C) 2009 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.primitives;

import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtIncompatible;
import com.google.common.annotations.VisibleForTesting;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import java.nio.ByteOrder;
import java.util.Comparator;
import sun.misc.Unsafe;

/**
 * Static utility methods pertaining to {@code byte} primitives that interpret values as
 * <i>unsigned</i> (that is, any negative value {@code b} is treated as the positive value
 * {@code 256 + b}). The corresponding methods that treat the values as signed are found in
 * {@link SignedBytes}, and the methods for which signedness is not an issue are in {@link Bytes}.
 *
 * <p>
 * See the Guava User Guide article on
 * <a href="https://github.com/google/guava/wiki/PrimitivesExplained">primitive utilities</a>.
 *
 * @author Kevin Bourrillion
 * @author Martin Buchholz
 * @author Hiroshi Yamauchi
 * @author Louis Wasserman
 * @since 1.0
 */
@GwtIncompatible
public final class UnsignedBytes {
    private UnsignedBytes() {}

    /**
     * The largest power of two that can be represented as an unsigned {@code
     * byte}.
     *
     * @since 10.0
     */
    public static final byte MAX_POWER_OF_TWO = (byte) 0x80;

    /**
     * The largest value that fits into an unsigned byte.
     *
     * @since 13.0
     */
    public static final byte MAX_VALUE = (byte) 0xFF;

    private static final int UNSIGNED_MASK = 0xFF;

    /**
     * Returns the value of the given byte as an integer, when treated as unsigned. That is, returns
     * {@code value + 256} if {@code value} is negative; {@code value} itself otherwise.
     *
     * @since 6.0
     */
    public static int toInt(byte value) {
        return value & UNSIGNED_MASK;
    }

    /**
     * Returns the {@code byte} value that, when treated as unsigned, is equal to {@code value}, if
     * possible.
     *
     * @param value a value between 0 and 255 inclusive
     * @return the {@code byte} value that, when treated as unsigned, equals {@code value}
     * @throws IllegalArgumentException if {@code value} is negative or greater than 255
     */
    @CanIgnoreReturnValue
    public static byte checkedCast(long value) {
        checkArgument(value >> Byte.SIZE == 0, "out of range: %s", value);
        return (byte) value;
    }

    /**
     * Returns the {@code byte} value that, when treated as unsigned, is nearest in value to
     * {@code value}.
     *
     * @param value any {@code long} value
     * @return {@code (byte) 255} if {@code value >= 255}, {@code (byte) 0} if {@code value <= 0},
     *         and {@code value} cast to {@code byte} otherwise
     */
    public static byte saturatedCast(long value) {
        if (value > toInt(MAX_VALUE)) {
            return MAX_VALUE; // -1
        }
        if (value < 0) {
            return (byte) 0;
        }
        return (byte) value;
    }

    /**
     * Compares the two specified {@code byte} values, treating them as unsigned values between 0
     * and 255 inclusive. For example, {@code (byte) -127} is considered greater than
     * {@code (byte) 127} because it is seen as having the value of positive {@code 129}.
     *
     * @param a the first {@code byte} to compare
     * @param b the second {@code byte} to compare
     * @return a negative value if {@code a} is less than {@code b}; a positive value if {@code a}
     *         is greater than {@code b}; or zero if they are equal
     */
    public static int compare(byte a, byte b) {
        return toInt(a) - toInt(b);
    }

    /**
     * Returns the least value present in {@code array}.
     *
     * @param array a <i>nonempty</i> array of {@code byte} values
     * @return the value present in {@code array} that is less than or equal to every other value in
     *         the array
     * @throws IllegalArgumentException if {@code array} is empty
     */
    public static byte min(byte... array) {
        checkArgument(array.length > 0);
        int min = toInt(array[0]);
        for (int i = 1; i < array.length; i++) {
            int next = toInt(array[i]);
            if (next < min) {
                min = next;
            }
        }
        return (byte) min;
    }

    /**
     * Returns the greatest value present in {@code array}.
     *
     * @param array a <i>nonempty</i> array of {@code byte} values
     * @return the value present in {@code array} that is greater than or equal to every other value
     *         in the array
     * @throws IllegalArgumentException if {@code array} is empty
     */
    public static byte max(byte... array) {
        checkArgument(array.length > 0);
        int max = toInt(array[0]);
        for (int i = 1; i < array.length; i++) {
            int next = toInt(array[i]);
            if (next > max) {
                max = next;
            }
        }
        return (byte) max;
    }

    /**
     * Returns a string representation of x, where x is treated as unsigned.
     *
     * @since 13.0
     */
    @Beta
    public static String toString(byte x) {
        return toString(x, 10);
    }

    /**
     * Returns a string representation of {@code x} for the given radix, where {@code x} is treated
     * as unsigned.
     *
     * @param x the value to convert to a string.
     * @param radix the radix to use while working with {@code x}
     * @throws IllegalArgumentException if {@code radix} is not between {@link Character#MIN_RADIX}
     *         and {@link Character#MAX_RADIX}.
     * @since 13.0
     */
    @Beta
    public static String toString(byte x, int radix) {
        checkArgument(radix >= Character.MIN_RADIX && radix <= Character.MAX_RADIX,
                "radix (%s) must be between Character.MIN_RADIX and Character.MAX_RADIX", radix);
        // Benchmarks indicate this is probably not worth optimizing.
        return Integer.toString(toInt(x), radix);
    }

    /**
     * Returns the unsigned {@code byte} value represented by the given decimal string.
     *
     * @throws NumberFormatException if the string does not contain a valid unsigned {@code byte}
     *         value
     * @throws NullPointerException if {@code string} is null (in contrast to
     *         {@link Byte#parseByte(String)})
     * @since 13.0
     */
    @Beta
    @CanIgnoreReturnValue
    public static byte parseUnsignedByte(String string) {
        return parseUnsignedByte(string, 10);
    }

    /**
     * Returns the unsigned {@code byte} value represented by a string with the given radix.
     *
     * @param string the string containing the unsigned {@code byte} representation to be parsed.
     * @param radix the radix to use while parsing {@code string}
     * @throws NumberFormatException if the string does not contain a valid unsigned {@code byte}
     *         with the given radix, or if {@code radix} is not between {@link Character#MIN_RADIX}
     *         and {@link Character#MAX_RADIX}.
     * @throws NullPointerException if {@code string} is null (in contrast to
     *         {@link Byte#parseByte(String)})
     * @since 13.0
     */
    @Beta
    @CanIgnoreReturnValue
    public static byte parseUnsignedByte(String string, int radix) {
        int parse = Integer.parseInt(checkNotNull(string), radix);
        // We need to throw a NumberFormatException, so we have to duplicate checkedCast. =(
        if (parse >> Byte.SIZE == 0) {
            return (byte) parse;
        } else {
            throw new NumberFormatException("out of range: " + parse);
        }
    }

    /**
     * Returns a string containing the supplied {@code byte} values separated by {@code separator}.
     * For example, {@code join(":", (byte) 1, (byte) 2,
     * (byte) 255)} returns the string {@code "1:2:255"}.
     *
     * @param separator the text that should appear between consecutive values in the resulting
     *        string (but not at the start or end)
     * @param array an array of {@code byte} values, possibly empty
     */
    public static String join(String separator, byte... array) {
        checkNotNull(separator);
        if (array.length == 0) {
            return "";
        }

        // For pre-sizing a builder, just get the right order of magnitude
        StringBuilder builder = new StringBuilder(array.length * (3 + separator.length()));
        builder.append(toInt(array[0]));
        for (int i = 1; i < array.length; i++) {
            builder.append(separator).append(toString(array[i]));
        }
        return builder.toString();
    }

    /**
     * Returns a comparator that compares two {@code byte} arrays
     * <a href="http://en.wikipedia.org/wiki/Lexicographical_order">lexicographically</a>. That is,
     * it compares, using {@link #compare(byte, byte)}), the first pair of values that follow any
     * common prefix, or when one array is a prefix of the other, treats the shorter array as the
     * lesser. For example, {@code [] < [0x01] < [0x01, 0x7F] < [0x01, 0x80] < [0x02]}. Values are
     * treated as unsigned.
     *
     * <p>
     * The returned comparator is inconsistent with {@link Object#equals(Object)} (since arrays
     * support only identity equality), but it is consistent with
     * {@link java.util.Arrays#equals(byte[], byte[])}.
     *
     * @since 2.0
     */
    public static Comparator<byte[]> lexicographicalComparator() {
        return LexicographicalComparatorHolder.BEST_COMPARATOR;
    }

    @VisibleForTesting
    static Comparator<byte[]> lexicographicalComparatorJavaImpl() {
        return LexicographicalComparatorHolder.PureJavaComparator.INSTANCE;
    }

    /**
     * Provides a lexicographical comparator implementation; either a Java implementation or a
     * faster implementation based on {@link Unsafe}.
     *
     * <p>
     * Uses reflection to gracefully fall back to the Java implementation if {@code Unsafe} isn't
     * available.
     */
    @VisibleForTesting
    static class LexicographicalComparatorHolder {
        static final String UNSAFE_COMPARATOR_NAME =
                LexicographicalComparatorHolder.class.getName() + "$UnsafeComparator";

        static final Comparator<byte[]> BEST_COMPARATOR = getBestComparator();

        @VisibleForTesting
        enum UnsafeComparator implements Comparator<byte[]> {
            INSTANCE;

            static final boolean BIG_ENDIAN = ByteOrder.nativeOrder().equals(ByteOrder.BIG_ENDIAN);

            /*
             * The following static final fields exist for performance reasons.
             *
             * In UnsignedBytesBenchmark, accessing the following objects via static final fields is
             * the fastest (more than twice as fast as the Java implementation, vs ~1.5x with
             * non-final static fields, on x86_32) under the Hotspot server compiler. The reason is
             * obviously that the non-final fields need to be reloaded inside the loop.
             *
             * And, no, defining (final or not) local variables out of the loop still isn't as good
             * because the null check on the theUnsafe object remains inside the loop and
             * BYTE_ARRAY_BASE_OFFSET doesn't get constant-folded.
             *
             * The compiler can treat static final fields as compile-time constants and can
             * constant-fold them while (final or not) local variables are run time values.
             */

            static final Unsafe theUnsafe;

            /** The offset to the first element in a byte array. */
            static final int BYTE_ARRAY_BASE_OFFSET;

            static {
                theUnsafe = getUnsafe();

                BYTE_ARRAY_BASE_OFFSET = theUnsafe.arrayBaseOffset(byte[].class);

                // sanity check - this should never fail
                if (theUnsafe.arrayIndexScale(byte[].class) != 1) {
                    throw new AssertionError();
                }
            }

            /**
             * Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package. Replace with a
             * simple call to Unsafe.getUnsafe when integrating into a jdk.
             *
             * @return a sun.misc.Unsafe
             */
            private static sun.misc.Unsafe getUnsafe() {
                try {
                    return sun.misc.Unsafe.getUnsafe();
                } catch (SecurityException e) {
                    // that's okay; try reflection instead
                }
                try {
                    return java.security.AccessController
                            .doPrivileged(new java.security.PrivilegedExceptionAction<sun.misc.Unsafe>() {
                                @Override
                                public sun.misc.Unsafe run() throws Exception {
                                    Class<sun.misc.Unsafe> k = sun.misc.Unsafe.class;
                                    for (java.lang.reflect.Field f : k.getDeclaredFields()) {
                                        f.setAccessible(true);
                                        Object x = f.get(null);
                                        if (k.isInstance(x)) {
                                            return k.cast(x);
                                        }
                                    }
                                    throw new NoSuchFieldError("the Unsafe");
                                }
                            });
                } catch (java.security.PrivilegedActionException e) {
                    throw new RuntimeException("Could not initialize intrinsics", e.getCause());
                }
            }

            @Override
            public int compare(byte[] left, byte[] right) {
                int minLength = Math.min(left.length, right.length);
                int minWords = minLength / Longs.BYTES;

                /*
                 * Compare 8 bytes at a time. Benchmarking shows comparing 8 bytes at a time is no
                 * slower than comparing 4 bytes at a time even on 32-bit. On the other hand, it is
                 * substantially faster on 64-bit.
                 */
                for (int i = 0; i < minWords * Longs.BYTES; i += Longs.BYTES) {
                    long lw = theUnsafe.getLong(left, BYTE_ARRAY_BASE_OFFSET + (long) i);
                    long rw = theUnsafe.getLong(right, BYTE_ARRAY_BASE_OFFSET + (long) i);
                    if (lw != rw) {
                        if (BIG_ENDIAN) {
                            return UnsignedLongs.compare(lw, rw);
                        }

                        /*
                         * We want to compare only the first index where left[index] !=
                         * right[index]. This corresponds to the least significant nonzero byte in
                         * lw ^ rw, since lw and rw are little-endian.
                         * Long.numberOfTrailingZeros(diff) tells us the least significant nonzero
                         * bit, and zeroing out the first three bits of L.nTZ gives us the shift to
                         * get that least significant nonzero byte.
                         */
                        int n = Long.numberOfTrailingZeros(lw ^ rw) & ~0x7;
                        return ((int) ((lw >>> n) & UNSIGNED_MASK)) - ((int) ((rw >>> n) & UNSIGNED_MASK));
                    }
                }

                // The epilogue to cover the last (minLength % 8) elements.
                for (int i = minWords * Longs.BYTES; i < minLength; i++) {
                    int result = UnsignedBytes.compare(left[i], right[i]);
                    if (result != 0) {
                        return result;
                    }
                }
                return left.length - right.length;
            }

            @Override
            public String toString() {
                return "UnsignedBytes.lexicographicalComparator() (sun.misc.Unsafe version)";
            }
        }

        enum PureJavaComparator implements Comparator<byte[]> {
            INSTANCE;

            @Override
            public int compare(byte[] left, byte[] right) {
                int minLength = Math.min(left.length, right.length);
                for (int i = 0; i < minLength; i++) {
                    int result = UnsignedBytes.compare(left[i], right[i]);
                    if (result != 0) {
                        return result;
                    }
                }
                return left.length - right.length;
            }

            @Override
            public String toString() {
                return "UnsignedBytes.lexicographicalComparator() (pure Java version)";
            }
        }

        /**
         * Returns the Unsafe-using Comparator, or falls back to the pure-Java implementation if
         * unable to do so.
         */
        static Comparator<byte[]> getBestComparator() {
            try {
                Class<?> theClass = Class.forName(UNSAFE_COMPARATOR_NAME);

                // yes, UnsafeComparator does implement Comparator<byte[]>
                @SuppressWarnings("unchecked")
                Comparator<byte[]> comparator = (Comparator<byte[]>) theClass.getEnumConstants()[0];
                return comparator;
            } catch (Throwable t) { // ensure we really catch *everything*
                return lexicographicalComparatorJavaImpl();
            }
        }
    }
}
